1. Field of the Invention
The present invention relates to a method for manufacturing a capacitor of a semiconductor element, and more particularly to a method for manufacturing a capacitor of a semiconductor element, which achieves a high capacitance using dielectric films made of Al2O3 and HfO2 having a high dielectric constant.
2. Description of the Related Art
In order to achieve high-integration of a semiconductor element, methods for decreasing the dimensions of cells and lowering operating voltage have been researched and developed. Further, as a semiconductor element becomes increasingly integrated, the dimensions of a capacitor rapidly decrease, but an electric charge required for operating a memory element, i.e., capacitance per unit area, must be increased.
In order to assure sufficient capacitance of the capacitor, research into thinning a dielectric film, developing the structure for increasing the effective surface area of a capacitor electrode, and using a high dielectric material, such as Ta2O5, BST(BaSrTiO3), and Al2O3, in substitute for an nitride-oxide (NO) or an oxide-nitride-oxide (ONO) structure, as a dielectric film serving as an oxide film, are currently underway.
In order to reduce the increase of leakage current generated when the thickness of a dielectric film of the capacitor, the dielectric film does not use a single film, but uses a film containing several materials having a high dielectric constant. For example, a film made of Ta2O5/TiO2, Al2O3/TiO2, Al2O3/HfO2, Al2O3/ZrO2, Ta2O5/HfO2, and Ta2O5/ZrO2 is used as the dielectric film. Particularly, a composite dielectric film, having a double-layered or multi-layered structure, containing Al2O3, which has a low dielectric constant of 10 but a high leakage current prevention effect, and HfO2, which has a high dielectric constant of 20˜25 and high leakage current prevention effect due to a high band gap, has been developed and researched.
FIG. 1 is a schematic longitudinal sectional view of a conventional capacitor having Al2O3 and HfO2 dielectric films of a semiconductor element.
With reference to FIG. 1, the conventional capacitor having Al2O3 and HfO2 dielectric films has a silicon insulator silicon (SIS) structure, and comprises a bottom electrode 10, made of doped polysilicon, formed under a semiconductor substrate, and an Al2O3 dielectric film 14 and an HfO2 dielectric film 16, which serve as a composite dielectric film, sequentially formed on the bottom electrode 10. The conventional capacitor further comprises a plate electrode 18, made of doped polysilicon, formed on the Al2O3 and HfO2 dielectric films 14 and 16. Here, a Si3N4 film 12 is formed between the bottom electrode 10 and the Al2O3 and HfO2 dielectric films 14 and 16.
In order to prevent the generation of a natural oxide film between the bottom electrode 10 and the Al2O3 film 14 of the conventional capacitor, a rapid thermal nitrification (RTN) process is performed. Thereafter, nitrification for reducing a thermal budget of the plate electrode 18 to prevent the plate electrode 18 from reacting with HfO2 is performed.
When the total thickness of the Al2O3 and HfO2 dielectric films 14 and 16 is decreased to approximately 25 Å by the above nitrification, leakage current is also lowered. Accordingly, the capacitor having Al2O3 and HfO2 dielectric films has a capacitance higher than that of a capacitor having an Al2O3 dielectric film.
However, in the case that the RTN (for example, at a temperature of 800˜900° C.) is performed on the surface of the bottom electrode, stress is applied to a gate electrode of a semiconductor substrate, and then edge of an isolation film. When a capacitor is manufactured by forming Al2O3 and HfO2 dielectric films on the semiconductor element, to which the above stress is applied, the manufactured capacitor has a desired high capacitance, but is disadvantageous in that the force needed for driving a transistor, such as refresh, is deteriorated.